The Remote Control (RC) car or truck gearing ratio information is available at Sources and Citations.
1Tighten the gear mesh so it is not fitted too tightly, or else it will make noise and cause premature wear on both the pinion and spur gear. Placing a piece of notebook paper between the pinion and spur before tightening down the motor will aid in setting the gear mesh. After the motor is secure, remove the paper and check the gear mesh. You should have minute amount of play between the gears.
2Listen for noise: if you have it set correctly you should not hear any noise coming from the gears. It is too loose it will cause gears to wear also, or to slip and clash and ruin the mesh.
3Try to use a narrow strip of printer or copier paper by putting it between the pinion and spur gear while you set the tightness.
4Adjust the pinion to the spur with the paper between them, and then tighten the motor mounting screws, and you should be able to pull/work the paper out without it tearing it off.
5A smaller pinion (and/or larger spur) will give you faster acceleration, less load on the motor, and slower top speed. A larger pinion (and/or smaller spur) will slow the acceleration rate, increase the the load on the motor, and increase the top speed. The greater the load = the hotter your motor will get. The trick is to not overload (overheat) the motor. You should be able to hold on to the motor after a 4-6 minute run. If you can't hold on to it ( approx 130 degree F or higher ) your putting to much load on the motor and should decrease the pinion size or increase the spur.
That's a simple explanation. Hobbyists and racers can spend a lot of time trying to figure out the perfect balance of gears to give enough speed but still have enough torque for fast acceleration. We take into account tire size, track conditions, traction of the track and tires, duration of the race, motor types, battery specs, etc.
Basically, it's trial and error. Good luck!